Abstract

Abstract
The system TiO2-SnO2 has been studied experimentally at pressures of 0-6.5 GPa and temperatures of 1020-1430-C. The extent of solid solution in the rutile structure was found to increase with pressure; the temperature of the solvus crest was fit as Tcr (ºC) = 1430 - 61.9 P (GPa). The data are consistent with a symmetrical regular solution model. The Margules parameter was fit as a linear function of pressure, W (kJ/ mol) = 28.92 - 1.03 P (GPa). However, theoretical solid solution models suggest that the non-ideality should increase with pressure, which would be consistent with the observation that the volume of mixing at atmospheric pressure is positive. Therefore, the Margules parameter would be expected to become more positive, and the solvus gap to widen, with increasing pressure. The behaviour observed may reflect convergence towards a metastable equilibrium state rather than the global minimum of free energy.
It is possible that the negative value derived for the apparent Vmix from the experimental data is due to 'solid solution hardening', i.e. the enhanced shear modulus of intermediate compositions relative to endmembers. The likelihood of such behaviour complicates considerably the analysis of high-pressure experimental data from solid solutions. Behaviour that appears to be internally consistent may nevertheless correspond to a metastable state rather than to true equilibrium.
Twinning on {101} in the rutile phase, and metastable transformation to the α-PbO2 phase were observed in some samples. This was predominantly isochemical, and appears to have occurred during cold, slightly non-hydrostatic pressurisation outside the true stability field of the alpha phase. Given that the atomic arrangement across a {101} twin boundary in rutile corresponds closely to that in untwinned α-PbO2, it is likely that the twins acted as nuclei for growth of the alpha phase. The observed orientation relationships between rutile structure matrix, twins and lamellae of alpha phase are consistent with diffusionless transformation via a fluorite- or baddeleyite-like transition state, as proposed elsewhere.